Conventionally, valve devices of different shapes, sizes and structures are proposed and used depending on the application, use conditions, operating conditions and so on. Among the wide variety of valve devices, large-sized valve devices are principally used in large facilities such as industrial plants or power plants having a large-scale piping system. As such large-sized valve devices, motor-operated valves which operate on electric power are usually employed in view of operability and operation reliability.
Such a motor-operated valve has a valve element for opening and closing a passage in a pipe, a worm rotatably driven by a motor or human power, a valve element driving part which receives torque from the worm to open and close the valve element, a spring cartridge in which disc springs are held in a compressed state by a prescribed compression force (the compression force is referred to as “preload”) and which can expand or contrast in proportion to a thrust generated in the worm. When the valve element is opened or closed, the torque is applied to the valve element driving part. Then, when the amount of compression of the spring cartridge reaches a predetermined value (which corresponds to “set torque”), the motor is stopped by a torque switch to ensure an appropriate open/close retention force on the valve element for reliable valve function and to prevent damage to any part caused by an excessively large driving force and overload on the motor.
In order to allow a motor-operated valve to maintain its original functions over a long period of time, a diagnosis should be made on various points. Above all, the diagnosis on the set torque, that is, the diagnosis on whether or not an appropriate open/close retention force is exerted on the valve element when it is in full open or close position, in other words, whether the compression force on the spring cartridge (which is proportional to torque) or the amount of compression of the spring cartridge (which depends on the compression force, that is, the torque) is maintained in an appropriate state is one of the most important point to be checked.
However, the spring characteristic of the spring cartridge (the correspondence relation between the compression force and the amount of compression) varies with time because, for example, of wear or a change in the alignment of the disc springs, or loosening of the nut at an end of the spring cartridge. Therefore, it is necessary to diagnose the set torque at times and determine its appropriateness. As methods for diagnosing the set torque, various methods have been proposed. One representative example is an “external torque sensor method” described in Patent Document 1 shown below (paragraphs [0021] to [0031], FIG. 1 to FIG. 4), and another representative example is a “spring compression method” described in Patent Document 2 shown below (paragraphs [0028] to [0036], FIG. 1 and FIG. 2). In both the methods, a measuring means for acquiring information on torque, that is, the compression force on the spring cartridge and the amount of compression of the spring cartridge, is attached to an outer end of the spring cartridge and a diagnosis on torque of the motor-operated valve including the set torque is conducted based on information acquired by the measuring means.
Patent Document 1 and Patent Document 2 herein are JP-B-2982090 and JP-A-H07-310845, respectively.